loader: remove open_disk cache

As we provide the disk size verification and correction via disk_ioctl
and disk state provided by disk_open(), we can not share the partition
state in disk_devdesc structure. Also the sharing does make a lot of sense
with ufs, as only one partition is open at any given time, but zfs pools
do keep the disk devices open.

To make sure we do get the correct information about the open device,
just remove the cache.

Reviewed by: allanjude, smh
Approved by: allanjude (mentor)
Differential Revision: https://reviews.freebsd.org/D9757